Network Working Group A. Mayrhofer Internet-Draft nic.at GmbH Intended status: Standards Track January 25, 2016 Expires: July 28, 2016 The EDNS(0) Padding Option draft-ietf-dprive-edns0-padding-02 Abstract This document specifies the EDNS(0) 'Padding' option, which allows DNS clients and servers to pad request and response messages by a variable number of octets. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on July 28, 2016. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Mayrhofer Expires July 28, 2016 [Page 1] Internet-Draft draft-ietf-dprive-edns0-padding January 2016 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. The 'Padding' Option . . . . . . . . . . . . . . . . . . . . 3 4. Usage Considerations . . . . . . . . . . . . . . . . . . . . 3 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 4 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 8.1. draft-ietf-dprive-edns0-padding-02 . . . . . . . . . . . 4 8.2. draft-ietf-dprive-edns0-padding-01 . . . . . . . . . . . 5 8.3. draft-ieft-dprive-edns0-padding-00 . . . . . . . . . . . 5 8.4. draft-mayrhofer-edns0-padding-01 . . . . . . . . . . . . 5 8.5. draft-mayrhofer-edns0-padding-00 . . . . . . . . . . . . 5 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 9.1. Normative References . . . . . . . . . . . . . . . . . . 5 9.2. Informative References . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction The Domain Name System (DNS) [RFC1035] was specified to transport DNS messages in clear text form. Since this can expose significant amounts of information about the internet activities of an end user, the IETF has undertaken work to provide confidentiality to DNS transactions (see the DPRIVE WG). Encrypting the DNS transport is considered as one of the options to improve the situation. However, even if both DNS query and response messages were encrypted, meta data of could still be used to correlate such messages with well known unencrypted messages, hence jeopardizing some of the confidentiality gained by encryption. One such property is the message size. This document specifies the Extensions Mechanisms for DNS (EDNS(0)) "Padding" Option, which allows to artificially increase the size of a DNS message by a variable number of bytes, significantly hampering size-based correlation of the encrypted message. 2. Terminology The terms "Requestor", "Responder" are to be interpreted as specified in [RFC6891]. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and Mayrhofer Expires July 28, 2016 [Page 2] Internet-Draft draft-ietf-dprive-edns0-padding January 2016 "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. The 'Padding' Option The EDNS(0) [RFC6891] specifies a mechanism to include new options in DNS packets, contained in the RDATA of the OPT meta-RR. This document specifies the 'Padding' option in order to allow clients and servers pad DNS packets by a variable number of bytes. The 'Padding' option MUST occur at most once per OPT meta-RR (and hence, at most once per message). The figure below specifies the structure of the option in the RDATA of the OPT RR: 0 8 16 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | OPTION-CODE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | OPTION-LENGTH | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | (PADDING) ... (PADDING) ... / +- - - - - - - - - - - - - - - - Figure 1 The OPTION-CODE for the 'Padding' option is 12. The OPTION-LENGTH for the 'Padding' option is the size (in octets) of the PADDING. The minimum number of padding octets is 0. The PADDING octets SHOULD be set to 0x00. Other values MAY be used; for example, in cases where there is a concern that the padded message could be subject to compression before encryption. PADDING octets of any value MUST be accepted in messages received. 4. Usage Considerations This document does not specify the actual amount of padding to be used, since this depends on the situation in which the option is used. However, padded DNS messages MUST NOT exceed the number of octets specified in the Requestor's Payload Size field encoded in The RR Class Field (see Section 6.2.3 and 6.2.4 of [RFC6891]). Responders MUST pad DNS responses when the respective DNS query included the 'Padding' option, unless doing so would violate the maximum UDP payload size. Mayrhofer Expires July 28, 2016 [Page 3] Internet-Draft draft-ietf-dprive-edns0-padding January 2016 Responders MAY pad DNS responses when the respective DNS query indicated EDNS(0) support of the Requestor. Responders MUST NOT pad DNS responses when the respective DNS query did not indicate EDNS(0). 5. IANA Considerations IANA has assigned EDNS Option Code 12 for Padding. IANA is requested to update the respective registration record by changing the Reference field to [[THISRFC]] and the Status field to 'Standard'. 6. Security Considerations Padding DNS packets obviously increases their size, and will therefore lead to increased traffic. The use of the EDNS(0) Padding provides only a benefit when DNS packets are not transported in clear text. Implementations therefore SHOULD avoid using this option if the DNS transport is not encrypted. Padding length might be affected by lower-level compression. Therefore (as described in Section 3.3 of [RFC7525]), implementations and deployments SHOULD disable TLS-level compression. The payload of the 'Padding' option could (like many other fields in the DNS protocol) be used as a covert channel. 7. Acknowledgements This document was inspired by a discussion with Daniel Kahn Gillmor during IETF93, as an alternative to the proposed padding on the TLS layer. Allison Mankin, Andreas Gustaffson, Christian Huitema and Jinmei Tatuya suggested text for this document. 8. Changes Note to RFC Editors: Please remove this whole section before publication 8.1. draft-ietf-dprive-edns0-padding-02 Clarified that changes section is to be removed before publication. Clarified that both Requestors and Responders are to ignore padding contents. changed text about non-zero padding contents based on WGLC comments. removed security considerations about truncation based on Mayrhofer Expires July 28, 2016 [Page 4] Internet-Draft draft-ietf-dprive-edns0-padding January 2016 WGLC comment. added more acknowledgements. replaced "packets" with "messages" where appropriate. 8.2. draft-ietf-dprive-edns0-padding-01 Fixed 'octects' typo. Changed 'covert channel' text to align with allowing non-0x00 padding. changed IANA considerations - assigned option code is 12. Changed field definitions to allow for non-0x00 padding, removed FORMERR requirement. referenced rfc7525 in security considerations. added acknowledgements. 8.3. draft-ieft-dprive-edns0-padding-00 Adopted by WG. Changed text about message size limit based on feedback. 8.4. draft-mayrhofer-edns0-padding-01 Changed minimum padding size to 0, rewrote Usage Considerations section, extended Security considerations section 8.5. draft-mayrhofer-edns0-padding-00 Initial version 9. References 9.1. Normative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013, . 9.2. Informative References Mayrhofer Expires July 28, 2016 [Page 5] Internet-Draft draft-ietf-dprive-edns0-padding January 2016 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, . Author's Address Alexander Mayrhofer nic.at GmbH Karlsplatz 1/2/9 Vienna 1010 Austria Email: alex.mayrhofer.ietf@gmail.com Mayrhofer Expires July 28, 2016 [Page 6]